Method for cleaning oil tanker holds

ABSTRACT

The hold in an oil tanker which has been emptied of its oil cargo is cleaned by first filling the hold with sea water ballast so that oil vapors are forced from the hold and a residual oil layer floats upon the sea water ballast. The floating oil layer is thereupon skimmed or otherwise removed from the aqueous ballast. The hold is then deballasted and washed with a jet of high pressure sea water.

United States Patent [1 1 Smith 1 July 17,1973

[ METHOD FOR CLEANING OIL TANKER HOLDS [75] Inventor: Billy E. Smith,Darien, Conn.

[73] Assignee: Gulf Oil Corporation, Pittsburgh, Pa.

[22] Filed: Mar. 22, 1972 [21] Appl. No.: 237,669

Related US. Application Data [63] Continuation of Ser. No. 70,046, Sept.8, 1970,

abandoned.

[52] US. Cl. 134/22 R, 134/24 [51] Int. Cl B63b 57/02, B08b 9/00 [58]Field'of Search 134/22 R, 10, 24,

[56] References Cited UNITED STATES PATENTS 1,806,740 5/1931 Butterworth134/24 1,849,932. 3/1932 Johnson et a1... 134/24 X 1,891,592 12/1932Fitzgerald 134/24 2,065,462 12/1936 Olsson 134/22 R 2,092,321 9/1937McFadden 134/40 X Primary Examiner-Morris O. Wolk Assistant ExaminerD.G. Millman AttorneyMeyer Neishloss et al.

[5 7] ABSTRACT The hold in an oil tanker which has been emptied of itsoil cargo is cleaned by first filling the'hold'with sea water ballast sothat oil vapors are forced from the hold and a residual oil layer floatsupon the sea water ballast. The floating oil layer is thereupon skimmedor otherwise removed from the aqueous ballast. The hold is thendeballasted and washed with a jet of high pressure sea water.

5 Claims, 7 Drawing Figures PATEMED JUL 74975 sum 2 or 5 FIG. 2

BALLAST WA TER PATENIEDJUL 1 m I SHEEI 3 0F 6 METHOD FOR CLEANING OILTANKER HOLDS This invention is a continuation of Ser. No. 70,046, filedSept. 8, 1970, now abandoned.

This invention relates to a method for cleaning the hold of an oiltanker transporting crude oil or refined products which preventsoccurrence of explosive conditions in the atmosphere of the hold.

In ocean-going crude oil tanker vessels, the total storage space isdivided by bulkheads into a plurality of holds which are individuallyemptied of crude oil in port. Subsequently, either while still at portor on return voyage, before filling a hold with sea water ballast thehold is washed with one or several high pressure sea water jets whichare vertically and rotatably maneuverable within the hold to remove thefilm of oil which adheres to the bulkheads and to remove the oil residueremaining on the bottom of the hold.

The washing operation involves considerable danger of explosion becauseof combustible vapors emanating from the filmof liquid oil remaining onthe side bulkheads of the hold and from the small oil reservoirremaining on the bottom of the hold. Combustible vapors ordinarilyemanate from this oil residue but the quantity of these vapors isenhanced by the atomization action of high pressure washing jets. In anemptied hold the mixture of oil vapor and air is generally sufficientlyrich to be explosive even prior to sea water guncleaning but thecondition becomes aggravated by the agitation of guncleaning. The waterrequired for effective gunspray cleaning is generally more than 100pounds per square inch in pressure and the static electricity generatedby washing with jets of sea water at this pressure might be capable ofinducing a static spark whereby the danger of explosion is considerable.

Prior art methods have attempted to alleviate the danger of explosion bysearching for methods of washing which avoid induction of a spark.However, this approach not only results in inferior washing and prolongsthe duration of the wash cycle but also frequently requires that theship devoid of cargo remain in port during the washing operation andtherefore involves a great economic waste.

The present invention approaches the washing problem in a totallydifferent manner, i.e., it retains the existing washing method utilizinghigh pressure sea water jets which provides highly effective cleaningand permits the washing operation to be performed at sea. However,according to the present invention, combustible or explosive vaporconditions within the hold are avoided so that effective cleaning canproceed unhampered by concern of sparking or explosion. The danger ofexplosive conditions is obviated in accordance with the presentinvention by removing the oil residue 'remaining in the hold afteremptying of the crude oil cargo by completely filling the hold followingcrude oil removal therefrom with an aqueous ballast, such as sea water,causing essentially the entire oil film and tank floor reservoir of oilto float on the top fo the ballast in the region of the top of the hold.Thereupon, the oil layer is substantially completely skimmed, siphonedor aspirated from the top of the hold so that upon subsequent emptyingof the aqueous ballast, i.e., deballasting, the amount of oil coatingthe bulkheads and bottom of the hold is too low to form a combustible orexplosive mixture with air, whereby effective high pressure guncleaningcan proceed without concern about explosion, regardless of possiblespark conditions. Although sparking is no longer a problem, the removalof oil as described permits the extra precaution of effectivegun-cleaning with clean, cold sea or fresh water, while avoiding thehigh spark-inducing temperatures present during hot water or steamwashing.

In accordance with the present invention, the removal of oil residuefrom an empty tank or hold, as described, results in removal of thesource of combustible vapor. It. is emphasized that ballasting the tankwith sea water without removing the resulting oil layer is notproductive of the advantages of the present invention because uponremoval of the ballast the unstripped oil will recoat the tank walls andredeposit on the tank bottom. Therefore, the bulkheads and internals arecoated with oil and the tank bottom is the recipient of the residue asthough ballasting had never occurred. By ballasting and then skimming orotherwise removing the oil from the top of the ballast tank threeimportant advantages are realized. First, all gas in the tank isexpelled when the ballast is pressed to deck level. Secondly, when thetank is deballasted no oil remains .on the water surface to recoat thetank and settle on the bottom to provide a source of gas regeneration.Thirdly, since the skimming operation results in an 80, 90 or even 95percent or more removal of oil residue, the subsequent cleaningoperation is much more efficient and of shorter duration due to thereduced quantity of oil to be removed.

In an actual test in a 130,000 barrel supertanker ship hold, the removalof crude oil cargo and filling of the empty hold with sea water ballastresulted in a 10 inch layer of crude oil on the ballast surface prior toskimming, which is approximately 1,000 barrels. The cargo was Kuwaitcrude having a gravity of about 34 A.P.I. Other tests showed oil residuein a hold is about 0.5 to 1 percent depending on tank size andconfiguration. However, the surface oil layer floating on the ballastwas found to be considerably more viscous and had a gravity of aboutl4.7 A.P.I. Following siphoning off of the oil layer and subsequentdeballasting, the holds were guncleaned with to pounds per square inchjets of sea water and made sludge free. Tests of the tank atmospheresduring the water jet cleaning operation in accordance with thisinvention showed that the vapor concentrations were less than about 15per cent of the lower explosive limit. In the absence of skimming orsiphoning the supernatant oil layer, the vapor concentration in the tankatmospheres reached l00 percent of the lower explosive limit duringsimilar sea water jet cleaning.

The present invention will be more completely understood by reference tothe accompanying drawings wherein:

FIG. 1 is a cross-sectional view of a crude oil carrying vesselcontaining an apparatus of the present invention,

FIGS. 2, 3, 4 and 5 illustrate particular embodiments of oil skimmingapparatus of the present invention, and

FIGS. 6 and 7 illustrate graphically the improvement in atmosphericconditions within the hold of a vessel when employing the process andapparatus of the present invention.

Referring to FIG. 1, vessel 10 is shown in crosssection containing aplurality of holds including central hold 12 demarked by bulkheads 14and 16. Central hold 12 is filled with ballast water and an oil layerfloats on the ballast water in the upper region of the hold.

Also shown is a sea water jet apparatus 18 of which about three areordinarily employed in a hold. Gunspray cleaning apparatus 18 comprisesa shaft 20 internally through which motor 19 drives ball-like member 26on which is mounted nozzle 24. Ball-like member 26 rotates in thegeneral plane indicated by arrow 22 while nozzle 24 rotates in thegeneral plane indicated by either arrow 28. Arrows 22 and 28 indicaterotation on substantially transverse planes so that sea water spray 52is directed throughout the length and breadth of hold 12 in the patternof a figure 8, under a. pressure of 90-180 psi.

The oil surface indicated at 30 is provided with a floating skimmingdevice indicated generally at 32 and described more fully in FIG. 2.Skimming device 32 removes oil by siphoning through line 34 and pump 36.

Oil is initially pumped into hold 12 through port 42 by means of pump 38and line 40. The oil can be removed through port 44 by means of pump 46and line 48. Thereupon, hold 12 can be filled with ballast sea waterthrough the aforementioned pump 38, line 40 and port 42. The ballast seawater forces out of the hold an atmosphere of vapors which may be abovethe lower explosive limit. An oil layer 30 forms and floats upon theballast water and is skimmed or siphoned off by means of skimmingapparatus 32 in the manner explained below. After the oil layer isremoved, relatively oil-free ballast water is pumped out, water jetapparatus 18 is put into operation and the bulkheads 14 and 16 as wellas the base 50 of hold 12 are scrubbed clean of oil by means of a 90 to180 pounds per square inch pressure jet of cold or warm sea water 52.During the water scrubbing operation the oil vapor level within hold 12is well below the lower explosive limit.

Oil skimming apparatus 32 is illustrated more particularly in FIG. 2. Asshown in FIG. 2, the skimming apparatus permits oil layer 30 to overflowinto a funnel shaped fabric container 54, such as a canvas container,which container is supported by a steel ring 57 and small hooks 59. Anair-inflated plastic tube 61 is attached to the funnel assembly by oneor more clamps 58 in order to float the assembly at the oil surface. Aplurality of steel supports 56 are attached at one end to steel ring 57and on the other end to hinges 60 mounted on a base conduit 62 whichfeeds a suction hose 64 leading to pump 36, shown in FIG. 1. Pump 36 isprovided with'suitable priming means, not shown.

In the operation of the apparatus of FIG. 2, oil in the layer 30overflows into the canvas funnel 54 and flows downwardly between thecanvas and tube 61 for removal by means of hose 64. As the oil layer 30diminishes, the apparatus 32 progressively lowers until removal of seawater is observed at which time the siphoning operation is terminatedand the apparatus 32 is removed in preparation for deballasting thehold.

FIG. 3 shows a modification of the siphoning apparatus of FIG. 2. In theapparatus of FIG. 3 the steel supports are omitted and a funnel shapedcanvas 66 is clamped to a neoprene air-inflated circular tube 68,'or toother circular floatable tube means having a central opening, by meansof a plurality of clamps 70. The clamps 70 form a plurality ofpassageway weirs 72 in the tube through which oil at surface 30overflows into funnel 66.

FIG. 4 illustrates another apparatus for removing the oil layer from theballast water. Also illustrated in FIG. 4 is an apparatus forestablishing the depth of the oil layer. As indicated in FIG. 4, inputof ballast water forces the oil layer flush against the bottom of deck74 which is provided with an opening covered by cover plate 76 clampedsecurely into position by means of rotatable clamps 78. A centralstandpipe 80 projects from a central opening 82 in the cover plate,while lateral standpipes 84 project from lateral openings 86 in thecover plate.

A dip pipe 88 extends through the cover plate and is provided withmeasurement markings 90. The dip pipe 88 is provided with an exhaustpump, not shown, which siphons liquid through sight glass 89 and whichis provided with priming means. If dip pipe 88 were inserted throughopening 82 and removed, it is apparent that no matter when length ofpipe extended below plate 76 the entire portion projecting below plate76 would be coated with crude oil upon removal and no determination ofoil depth could be made. However, by inserting dip pipe 88 through theoil layer into the ballast water and siphoning out ballast water whilegradually raising the dip pipe until oil is observed to be removedinstead of water in sight glass 89, the marking indicia 90 will providemeasurement of the depth of the oil layer. Similarly, if dip pipe 88 isgradually lowered through the oil layer while siphoning oil until waterstarts to be siphoned instead of oil, the depth of the oil layer can bemeasured by the position at which siphoning of water begins. 7

In the operation of the apparatus of FIG. 4 for removal of the oillayer, standpipes 84 are each provided with suction pumps, not shown. Asthe oil suction pumps remove the oil layer the void is continuouslyfilled with sea water charged by any suitable pumping means to keep theoil layer flush against cover plate 76. Oil removal is complete whenwater is observed to be removed through pipes 84 instead of oil,assuming the ship is tilted so that the oil pool is disposed in thelocality of cover plate 76, or assuming the vessel is in a relativelyeven position.

FIG. 5 shows an oil siphoning apparatus which is integral with andnon-removable from the hold of a vessel. As shown in FIG. 5, 94 is thedeck of a vessel and 96 is a tank bulkhead. A suction cradle assembly 92is permanently attached to bulkhead 96. Cradle assembly 92 is comprisedof enclosure 98 mounted on angle support 102. The oil-water interface isat the height of and can enter into enclosure 98. The suction devicecomprises a neoprene bouyant circular collar 100 which is open in thecenter and which is flexibly attached to an overhead high velocityeductor tube 104 to which a high speed water hose can be attached at 106to accomplish suction or aspiration of oil through the collar. Eductortube 104 must not be more than 28 feet above collar 100 and is flexiblyattached to collar 100 by means of conduits 108 and 110 and flexibleconnectors 112 and 114, permitting collar 100 to freely move within theconfines of cage 98 upon rolling of the vessel.

FIG. 6 is a graph showing the conditions of the atmosphere in the holdof a vessel from which the oil cargo has been removed when the method ofskimming or siphoning in accordance with the present invention is notemployed. The graph shows the oil vapor richness in the atmosphere inthe hold expressed as per cent of the lower explosive limit. The graphshows various occurrences in the hold including blower venting, naturalventing and sea water gun jet cleaning, and their effect upon the percent of lower explosive limit of the hold atmosphere. As shown at theleft of FIG. 6, during gunspray cleaning the hold reached the lowerexplosive limit and is removed from said limit by means of blowerventing. Natural venting resulted in a trend back to the lower explosivelimit. Subsequent periods of water jet cleaning utilizing water of100-130 pounds per square inch pressure charged through three jetsresulted in the rapid approach to or actual reaching of the lowerexplosive limit.

FIG. 7 illustrates the great improvement accomplished in accordance withthe present invention. The graph of FIG. 7 shows the atmosphericcondition during sea water gunspray cleaning in a hold which had beenemptied of oil cargo and then filled with ballast sea water to form anoverlying oil layer, which oil layer was removed in accordance with thepresent invention. Following removal of the oil layer, the ballast waterwas removed and the hold was cleaned by means of three jets of seawaterat a pressure of l00-l30 per square inch gauge. FIG. 7 shows that in theatmosphere of the hold at no time during the water spray cleaningprocedure of the present invention was even percent of the lowerexplosive limit reached. The graph of FIG. 7 is typical of samples takenat various depths and positions in the hold. This is in sharp contrastto the results shown in FIG. 6 wherein gunspray cleaning under similarconditions resulted in the rapid reaching of 100 percent of the lowerexplosive limit in the atmosphere in the hold.

Various changes and modifications may be made without departing from thespirit of this invention or the scope thereof as defined in thefollowing claims.

I claim:

1. A process for cleaning a hold in an oil tanker which has been emptiedof its oil cargo comprising filling said hold with aqueous ballastliquid before pressure spraying said hold so that oil vapors are forcedupwardly from the hold and an oil liquid layer derived from oil on thewalls and bottom of the hold floats upon the aqueous ballast liquid assaid liquid moves upwardly towards the upper region of the hold,removing from the hold said oil liquid layer from the aqueous ballastliquid in the upper region of the hold, removing the aqueous ballastliquid after said removal of said oil liquid layer so that the removedliquid oil cannot recoat the walls and bottom of the hold, and cleaningby pressure liquid spraying said hold with water after said removal ofthe aqueous ballast liquid.

2. The process of claim 1 wherein said oil liquid layer is removed byskimming.

3. The process of claim 1 wherein said oil liquid layer is removed bysiphoning.

4. The process of claim 1 wherein said oil liquid layer is removed byaspiration.

5. The process of claim 1 wherein the atmosphere in said hold is abovethe lower explosive limit prior to the aqueous ballast filling step andis below said limit as a result of said aqueous filling and oil removingsteps.

2. The process of claim 1 wherein said oil liquid layer is removed byskimming.
 3. The process of claim 1 wherein said oil liquid layer isremoved by siphoning.
 4. The process of claim 1 wherein said oil liquidlayer is removed by aspiration.
 5. The process of claim 1 wherein theatmosphere in said hold is above the lower explosive limit prior to theaqueous ballast filling step and is below said limit as a result of saidaqueous filling and oil removing steps.